We pulled up the stats on the 777 and we found that it holds only a few pallets less than the jumbo and the belly held more. As for the height, I know the 747 has a restriction on height as far as the nose loader goes and the pallet heights are restricted also due to the upper deck, at least for the first few rows of pallets. But with the fuel burns being so much less, the extra weight would have to be significant to to justify 4 engines vs. 2. At least that is my take.